Lymphocytes are one of several populations of white blood cells; they specifically recognize and respond to foreign antigen. The three major classes of lymphocytes are B lymphocytes (B cells), T lymphocytes (T cells) and natural killer (NK) cells. B lymphocytes are the cells responsible for antibody production and provide humoral immunity. B cells mature within the bone marrow and leave the marrow expressing an antigen-binding antibody on their cell surface. When a naive B cell first encounters the antigen for which its membrane-bound antibody is specific, the cell begins to divide rapidly and its progeny differentiate into memory B cells and effector cells called “plasma cells”. Memory B cells have a longer life span and continue to express membrane-bound antibody with the same specificity as the original parent cell. Plasma cells do not produce membrane-bound antibody but instead produce secreted form of the antibody. Secreted antibodies are the major effector molecules of humoral immunity.
The CD20 antigen (also called human B-lymphocyte-restricted differentiation antigen, Bp35) is a hydrophobic transmembrane protein with a molecular weight of approximately 35 kD located on pre-B and mature B lymphocytes (Valentine et al. J. Biol. Chem. 264(19):11282-11287 (1989); and Einfeld et al. EMBO J. 7(3):711-717 (1988)). The antigen is also expressed on greater than 90% of B cell non-Hodgkin's lymphomas (NHL) (Anderson et al. Blood 63(6):1424-1433 (1984)), but is not found on hematopoietic stem cells, pro-B cells, normal plasma cells or other normal tissues (Tedder et al. J. Immunol. 135(2):973-979 (1985)). CD20 is thought to regulate an early step(s) in the activation process for cell cycle initiation and differentiation (Tedder et al., supra) and possibly functions as a calcium ion channel (Tedder et al. J. Cell. Biochem. 14D:195 (1990)).
Given the expression of CD20 in B cell lymphomas, this antigen has been a useful therapeutic target to treat such lymphomas. There are more than 300,000 people in the United States with B-cell NHL and more than 56,000 new cases are diagnosed each year. For example, the rituximab (RITUXAN®) antibody which is a genetically engineered chimeric murine/human monoclonal antibody directed against human CD20 antigen (commercially available from Genentech, Inc., South San Francisco, Calif., U.S.) is used for the treatment of patients with relapsed or refractory low-grade or follicular, CD20 positive, B cell non-Hodgkin's lymphoma. Rituximab is the antibody referred to as “C2B8” in U.S. Pat. No. 5,736,137 issued Apr. 7, 1998 (Anderson et al.) and in U.S. Pat. No. 5,776,456. In vitro mechanism of action studies have demonstrated that RITUXAN® (rituximab) binds human complement and lyses lymphoid B cell lines through complement-dependent cytotoxicity (CDC) (Reff et al. Blood 83(2):435-445 (1994)). Additionally, it has significant activity in assays for antibody-dependent cellular cytotoxicity (ADCC). In vivo preclinical studies have shown that RITUXAN® (rituximab) depletes B cells from the peripheral blood, lymph nodes, and bone marrow of cynomolgus monkeys, presumably through complement and cell-mediated processes (Reff et al. Blood 83(2):435-445 (1994)). Other anti-CD20 antibodies indicated for the treatment of NHL include the murine antibody Zevalin™ (Ibritumomab Tiuxetan) which is linked to the radioisotope, Yttrium-90 (IDEC Pharmaceuticals, San Diego, Calif.), Bexxar™ (Tositumomab and Iodine|131 Tositumomab) which is a another fully murine antibody conjugated to I-131 (Corixa, Wash.).
A major limitation in the use of murine antibodies in human therapy is the human anti-mouse antibody (HAMA) response (see, e.g., Miller, R. A. et al. “Monoclonal antibody therapeutic trials in seven patients with T-cell lymphoma” Blood, 62:988-995, 1983; and Schroff, R. W., et al. “Human anti-murine immunoglobulin response in patients receiving monoclonal antibody therapy” Cancer Res., 45:879-885, 1985). Even chimeric molecules, where the variable (V) domains of rodent antibodies are fused to human constant (C) regions, are still capable of eliciting a significant immune response (HACA, human anti-chimeric antibody) (Neuberger et al. Nature (Lond.), 314:268-270, 1985). A powerful approach to overcome these limitations in the clinical use of monoclonal antibodies is “humanization” of the murine antibody or antibody from a non-human species (Jones et al. Nature (Lond), 321:522-525, 1986; Riechman et al., Nature (Lond), 332:323-327, 1988).
Thus, it is beneficial to produce therapeutic antibodies to the CD20 antigen that create minimal or no antigenicity when administered to patients, especially for chronic treatment. The present invention satisfies this and other needs. The present invention provides anti-CD20 antibodies that overcome the limitations of current therapeutic compositions as well as offer additional advantages that will be apparent from the detailed description below.